This invention relates to a lower limb prosthesis incorporating an adaptive prosthesis control system in the form of a processor-controlled knee flexion control unit which is arranged to resist the flexion at the knee joint in a variable way according to signals picked up from a sensing means on the prosthesis and processed in accordance with a stored program.
British Patent Application No. GB 2216426A discloses such a lower limb prosthesis. A pneumatic cylinder assembly is used as the knee flexion control unit to provide resistance to knee flexion and extension during the swing phase of the walking cycle. This unit includes a valve which is adjustable using a stepper motor to alter the degree of resistance according to signals received from a processor which senses walking speed, so that resistance to movement of the shin component about the knee axis is varied as the walking speed varies.
British Patent Application No. GB 2280609A teaches the programming of an adaptive prosthesis control system using a remote operator control unit allowing convenient setting up of the system by a prosthetist using a handheld remote control unit, whereby walking tests can be conducted at different speeds and a complete set of control data in which flexion resistance settings are mapped onto walking speed ranges can be quickly programmed. In the prosthesis described in Application No. GB 2280609A knee stability is achieved during the stance phase by means of a mechanical load-activated knee braking device consisting of an expandable brake shoe with a friction lining, housed in a brake drum.
One of the difficulties associated with the above-mentioned mechanical friction brake for knee stabilisation during the stance phase is its comparatively sharp transition from a condition in which the knee is free to flex to one in which it is locked. An alternative stance control device found in other lower limb prostheses makes use of a hydraulic piston and cylinder assembly which provides yielding resistance to flexion during the stance phase. This allows the amputee in many cases to achieve a more natural gait due to the ability to roll over the foot in mid stance while achieving smooth initial flexion at the knee. Extension of the knee prior to heel contact is assisted by a coiled compression spring inside the cylinder. However, the need to compress the spring during the latter part of the preceding stance phase leads to many amputees finding such an hydraulic swing and stance control unit tiring to use. The unit also tends to be difficult to adjust in many cases and the mechanism for controlling the onset and release of stance control can produce an unnatural gait.
It is an object of the present invention to provide improved control of knee flexion for a wide variety of amputees and in a wide variety of circumstances.
According to one aspect of this invention, a lower limb prosthesis for an above-knee amputee has an adaptive control system which comprises a knee flexion control device arranged to resist flexion at the knee joint both hydraulically and pneumatically. Sensor means may be provided for generating electrical sensor signals in response to loading of the prosthesis, together with an electronic processing circuit electrically coupled to the sensor means and the control device for automatically adjusting the hydraulic and pneumatic resistance to knee flexion according to the actions of the amputee, the system being arranged such that the resistance to knee flexion is controlled at least predominantly hydraulically during the stance phase of the walking cycle of the prosthesis and at least predominantly pneumatically during the swing phase of the walking cycle. Preferably, the system is configured such that knee flexion is hydraulically resisted substantially only within an initial part of the angular range of flexion of the knee from full extension, typically from zero to between 30xc2x0 and 35xc2x0 flexion. Conveniently, extension assistance can be provided pneumatically by the same control element which resists flexion during the swing phase, and terminal extension damping may be achieved hydraulically.
This advantageous combination may be provided in the form of a posteriorly mounted piston and cylinder assembly having two pistons mounted coaxially on a common piston rod and reciprocable together within respective chambers, one containing hydraulic fluid and the other containing a gas, e.g. air. In each case the flow of liquid or gas to or from the respective chambers may be restricted by valves automatically controlled in response to setting signals from the processing circuit whereby the resistance to flexion provided by the hydraulic part of the assembly is varied according to the mode of ambulation of the amputee (such as level walking, walking up or down an incline, walking up or down stairs, and so on), while resistance of the pneumatic part of the assembly is controlled according to the sensed speed of walking, as described in the above-mentioned prior published patent specifications. Arranging for the hydraulic part of the piston and cylinder assembly to resist flexion only during the first 30xc2x0 to 35xc2x0 of flexion limits the effect of the hydraulic part mainly to resisting flexion during the stance phase and, with appropriate directing of the hydraulic fluid, damping forward movement of the prosthesis shin part towards the end of the swing phase so as to cushion extension of the knee joint prior to terminal impact at the end of the swing phase.
Adjustment of a pneumatic piston and cylinder assembly according to walking speed by means of a needle valve driven by a stepper motor is described in detail in GB 2280609A. Compression of air against the needle valve during swing phase flexion, together with the comparatively small resistance provided by the hydraulic part of the assembly at large angles of flexion brings the advantage of efficiently generated extension assistance due to the compressed air cushion which exists at maximum flexion, obviating the need for a stiff extension assisting spring, as in prior hydraulic units. The preferred technique for controlling resistance to knee flexion during the stance phase using a piston in a chamber containing incompressible hydraulic fluid makes use of a variable orifice in a first bypass passage between ports in opposite end portions of the chamber and on opposite sides of the piston, the orifice area being controlled electromechanically using, for example, a stepper motor to move a valve member to different positions according to detected modes of amputee activity.
Provision of hydraulic and pneumatic resistance to flexion, both programmable according to activity mode, allows a wide range of adjustment which can be achieved precisely. For instance, coarse adjustment can be performed hydraulically and fine adjustment pneumatically. In this way, it is possible for the prosthetist to program the system to match the level of stance control to the residual muscular control available from the amputee.
The applicants have found that a particular improvement of the ease with which an above-knee amputee can descend a ramp can generally be obtained by increasing stance phase flexion resistance in comparison with the optimum setting for level walking. Consequently, in the preferred control system in accordance with this invention the sensor means and the processing circuit are operable together to generate a descent control signal indicative of the amputee walking down a slope or ramp, this descent control signal causing the stepper motor to move the valve member in the hydraulic bypass passage so as to restrict fluid flow to a greater degree than when the amputee is walking on the level.
Sensing of the ramp descent mode is advantageously performed by means of the processing circuit in combination with a sensor located at the knee level to provide a sensor output signal indicative of kinetic and kinematic parameters around the knee such as knee bending moment, i.e. the moment tending to cause knee flexion during the stance phase. In particular, the processing circuit is arranged to detect an increase in the bending moment around the prosthetic knee with reference to a stored reference level, the increase being sustained over a period of time which occupies at least the major part of the stance phase, thereby to produce the descent control signal. The applicants have found that the above bending moment during the stance phase when the amputee is descending the ramp, while not necessarily reaching a higher maximum value than during level walking, generally remains high over an extended period of time, to the extent that the descending ramp mode can be sensed by monitoring the integral of the bending moment with respect to time. Accordingly, when the value of this integral exceeds a reference level, the descent control signal is generated. In order to distinguish the ramp descent mode from the level walking mode at different speeds of walking, the integral is normalised with respect to the stance phase duration or the total step cycle period. In general, then, the force at the knee level is sensed over a period of time covering at least the major part of the stance phase, the resulting signals being used to determine a predetermined signal pattern.
Sensing of a bending moment related to loading of the limb may be performed by means of a force sensing resistor mounted between, for example, relatively movable parts in the region of the knee joint, and a particularly preferred arrangement is to place the force sensing resistor between, on the one hand, one end of a lever arm the other end of which is pivoted on a knee chassis member forming part of the thigh part of the prosthesis, and, on the other hand, a resilient element inserted between the lever arm and the knee chassis to provide small amounts of flexion independently of the knee flexion control device. Other transducers may be used for producing an electrical signal which is wholly or partly a function of the knee bending moment. For instance, a friction-operated force sensing resistor may be provided around the knee centre, or it may be mounted inside one of the chambers of the piston and cylinder assembly. Alternatively, a strain gauge may be fixed on the lever arm referred to above or on the piston rod connecting the piston and cylinder assembly to the thigh part or shin part of the prosthesis, or a strain gauge may be fixed to the cylinder body itself or on the upper part of the shin member, i.e. above the connection of the piston and cylinder assembly to the shin part. Yet a further alternative is the fitment of a pressure sensing cell inside the hydraulic fluid-containing chamber or as a load bearing member forming part of one of the pivotal joints between the piston and cylinder assembly and the thigh and the shin parts of the prosthesis.
A second sensor is preferably used for sensing the step period. Particular advantages are obtained by using a proximity sensor, e.g. a magnetoresistive transducer in combination with a permanent magnet mounted respectively on parts which move relative to each other when the knee is flexed. Thus, a magnetoresistor may be mounted on the body of the cylinder of the piston and cylinder assembly, while a permanent magnet is mounted on the piston rod or one of the pistons so that the distance between the magnet and the transducer varies from a minimum value to a maximum value when the knee is flexed between full extension and full flexion or vice versa. The proportional nature of the output of a magnetoresistive sensor lends itself to stroke position sensing, which maps directly onto flexion angle, albeit non-linearly, allowing not only the measurement of step period, but also the instantaneous stroke position or flexion angle value, as well as swing phase and stance phase durations.
In accordance with the invention, other activity modes can be detected. In all modes, but at least the cases of walking and descending an incline, the control system allows the prosthetist to set the hydraulic resistance to knee movement to suit the detected mode and to suit the ability of the amputee in each mode. Different aspects of the invention include detection of stairs descent using detection of a knee flexed period during the swing phase which is higher than that normally obtained during walking together with a peak flexion angle which exceeds a predetermined threshold. According to another aspect, a stumble condition can be detected in response to an abnormally short knee extended period, an abnormally increasing flexion angle during the stance phase, or an increasing flexion angle following a decreasing flexion angle whilst the knee is in a flexed state. According to a further aspect of the invention, a standing mode and a sitting mode can be detected respectively in response to the knee extended state or the knee flexed state being longer than a respective predetermined period.
According to another aspect of the invention, the electronic processing circuit is arranged such that automatic adjustment of the resistance to flexion in response to changes in the actions of the amputee are brought about by feeding a descent control signal to the control device in response to detection of the amputee descending an incline such that the descent control signal causes an increase in the resistance of the control device to knee flexion compared with the resistance set for level walking.
According to yet a further aspect of the invention the electronic processing circuit is. arranged automatically to adjust the resistance to knee flexion in response to changes in the sensor signals occurring when the amputee moves from walking on a level surface to walking down an incline and vice versa. In addition, adjustment of the hydraulic resistance to flexion may occur in response to changes in the sensor signals occurring when the amputee sits down, stands up, stops walking, and starts walking, by monitoring both the load on the prosthesis and the flexion angle with respect to a predetermined time period in the order of 2 to 7 seconds.
In the preferred knee flexion control device, independent hydraulic flexion and extension resistance is provided using dual bypass passages effective at low flexion angles, one passage containing a motorised valve for dynamic alteration of flexion resistance during the stance phase, and the other containing an adjustable valve which maybe manually operated for adjusting the stiffness of terminal extension damping at the end of the swing phase. In effect, the latter valve overrides the motorised valve on the extension stroke by appropriate direction of hydraulic fluid through the bypass passages using non-return valves.
A preferred embodiment of the invention will be described below with reference to the drawings. By combining both hydraulic and pneumatic control of knee movements in a single, compact control unit, with the movement in both the hydraulic and pneumatic parts of the unit regulated by microprocessor controlled electromagnetic actuators such as electric motors, a particularly versatile and effective control system is provided, allowing an amputee to achieve good gait characteristics not only during level walking at various speeds, but also on inclines. These benefits are obtained substantially without unnatural movements of the limb to trigger operation of the control system, and the prosthesis also offers advantages for standing, sitting, and negotiating stairs. The hydraulic flexion resistance, which provides knee stability during the stance phase, complements the amputee""s voluntary muscular knee stabilisation to a degree which is variable according to the amputee""s strength, dynamic variation being possible so that the level of stabilising assistance can be varied according to the mode of activity.